Photo-imaging utilizing alkali-activated photopolymerizable compositions

ABSTRACT

A photopolymerizable composition useful in photo-imaging processes comprises an ethylenically unsaturated polymerizable compound and a photo-initiator comprising a combination of a photoreducible dye and an alkanolamine. The composition is substantially insensitive to visible light when in an acidic pH condition, yet may be made highly sensitive to visible light upon adjustment to a condition of alkaline pH. Imaging materials may be repeatedly sensitized and desensitized by pH adjustment and multiple-spaced exposures made prior to final washoff development processing.

BACKGROUND

Numerous photopolymerizable coating compositions are known which may beused to make photo-imaging materials that are insensitive to light inthe visible region of the spectrum and which may, therefore, be handledunder conditions of ambient daylight or fluorescent illumination.Although preferred in copying processes, other materials which aresensitive to visible radiation must be protected from ambientillumination in order to prevent premature, indiscriminatepolymerization in the photosensitive composition. Dye sensitizedphotopolymerizable layers such as disclosed in U.S. Pat. Nos. 2,875,047and 3,573,922 may be handled under ambient illumination; however, suchmanipulation without safelight conditions must necessarily be limited tovery short periods in order to ensure against formation of non-imagepolymerization, or "fog".

Common to most photo-imaging systems is the requirement that thephotosensitive material be capable of inactivation subsequent toimagewise light exposure in order to "fix" the resulting image. Severalmethods of fixing have been previously employed, such as washing toremove active ingredients or deactivating such components with heat orchemical agents. The usual result of these procedures is that theimaging materials are limited to "one-shot" utility, since they arephotosensitive until a first imaging exposure and are thereafter "fixed"against any further imaging capability.

SUMMARY

The present invention provides photo-imaging material and a process ofphoto-imaging whereby a potentially photopolymerizable composition whichis substantially insensitive to visible light may be renderedtemporarily sensitive to such radiation in a simple activating step andafter imagewise light exposure will revert, without significantadditional treatment, to the normal inactive state. A particularadvantage in this invention is the continuing reversibility of thesensitivity of the composition which provides a means for applyingimages to a single sheet at widely spaced times, a capability which, forexample, finds great utility in record updating.

In accordance with the present invention, polymerization of vinylmonomers is accomplished by exposing a monomer or mixture of monomers toactinic radiation in the presence of a photoinitiator comprising aphotoreducible dye and an aromatic or aliphatic alkanolamine. Analkaline environment is required to support a useful rate ofpolymerization. For instance, acrylamide in aqueous solution with across-linking agent is converted to a hard, polymeric mass in a matterof seconds by exposure to light in the presence of such a photoinitiatorcomposition when the mixture is maintained on the alkaline side. Assmall incremental amounts of acid are added, the onset of polymerizationis considerably retarded until, at a pH of about 4.0 to 4.5, allpractical sensitivity to actinic radiation is lost.

Materials prepared by coating a layer of such an acidified compositionon a suitable support are insensitive to visible actinic radiation. Theymay be sensitized by adjusting the pH to the alkaline side by merelyfuming with moist ammonia as, for example, by passing a sheet throughthe developing chamber of a conventional diazo white-print machine. Animagewise exposure then made employing a 500-watt GE Photoflood lamp asthe light source effects polymerization in the light-struck portions ofthe coated composition. When next stored in darkness to allow theammonia to diffuse out of the coating, the sheet once again assumes itslight-insensitive state and thereafter can be handled under ambientlighting without adverse effects (fogging). Removal of ammonia from thecoating may be accelerated by mild heating, e.g., at 40° C for about oneminute. After this stabilizing step, the coating may again be sensitizedby fuming with moist ammonia, as before, and another imagewise exposuremade in previously non-exposed areas. After a plurality of imagewiseexposures have been made, each preceded by ammonia sensitization,remaining unexposed areas may be simply removed by a water wash at about30°-35° C to yield a like plurality of polymeric relief images, eachcorresponding to the previous respective imagewise light exposures. Ifthe mixture was not initially pre-pigmented or pre-dyed, the reliefimages are rendered distinctly visible by a post-dyeing step.

In procedures where it is desirable to repeatedly examine a record sheetduring the time span in which periodic image entries are beingaccumulated on the sheet, it is not practical to employ washoutdevelopment until such time as the sheet is imaged to spacial capacity.Record examination under such circumstances may, however, be veryeffectively accomplished through the use of a Schlieren optical system.The polymeric image is thus made readily visible for temporary perusalor the preparation of interim copies.

DESCRIPTION

Any normally liquid or solid photopolymerizable unsaturated compound isuseful in the practice of the present invention. Preferably, suchcompounds should be ethylenically unsaturated, i.e., contain at leastone non-aromatic double bond between adjacent carbon atoms. Compoundsparticularly advantageous include vinyl or vinylidene compoundscontaining a CH₂ =C< group activated by direct attachment to a negativegroup such a halogen, <C=O, --C.tbd.N, --C.tbd.C--, --O--, or aryl.Examples of photopolymerizable compounds include acrylamide, diacetoneacrylamide, N-methylolacrylamide, N-isopropyl acrylamide, acrylonitrile,acrylic acid, methacrylic acid, methacrylamide, N-vinyl pyrrolidone,N-vinyl carbazole, hydroxyethyl methacrylate, barium acrylate, bariummethacrylate, zinc acrylate, calcium acrylate, magnesium acrylate,itaconic acid esters, sodium or ammonium acrylates, and the like.

The ethylenically unsaturated compounds may be used either alone or inadmixture in order to vary the physical properties of the final polymer.Thus, in order to produce a polymer of the desired physicalcharacteristics, it is a recognized practice to polymerize the vinylmonomer in the presence of a small amount of an unsaturated compoundhaving at least two terminal vinyl groups each linked to a carbon atomin a straight chain or in a ring. The function of such compounds is tocross-link the polyvinyl chains. Cross-linking agents which can beutilized for the purposes described herein includeN,N'-methylenebisacrylamide, triallyl cyanurate, divinyl benzene,divinyl ketones, and diglycol diacrylate. Other difunctional monomerswhich can also be used advantageously include zinc, calcium, barium ormagnesium acrylates. Generally, increasing the quantity of cross-linkingagent increases the hardness of the polymer obtained. The quantity ofcross-linking agent may desirably range from about 2 to 10 parts for 100parts of monomer.

The photoreducible dyes which may be used as components of thephoto-initiator in the present invention are those dyes which arecapable of forming a stable system with an electron donor in the absenceof light and which will undergo reduction when irradiated with visiblelight in the presence of the electron donating component. Suchphotoreducible dyes includes rose bengal, erythrosine, eosin,fluorescein, acriflavine, thionin, methylene blue, riboflavin, and thelike. The dyes may be used individually or in admixture to increasesensitivity over a wider range of the visible spectrum.

While the concentration of dye is not critical, the most efficientresults are obtained when the concentration is adjusted so that at least90% of the incident light is absorbed at the wavelength corresponding tothe absorption maximum of the particular dye employed.

The alkanolamines used as the remaining component part of thephotocatalyst are very stable compounds, but are capable of reactingwith the photoexcited dyes (electron acceptors) to produce free radicalsby a redox reaction. It is apparently these photogenerated free radicalswhich initiate the polymerization reaction.

Aromatic and aliphatic alkanolamines suitable for the purposes of thisinvention include N-phenyldiethanolamine, triethanolamine,diethanolamine, monoethanolamine and derivatives thereof. One example ofa useful derivative is a coco amine adduct with 15 moles of ethyleneoxide, which is marketed by the Ashland Chemical Company as Arosurf160E-15. The coco amine in this case has a minimum primary amine contentof 95%. Another useful product is Polycomplex Q, a coconut fatty acidalkanolamide, manufactured by the Guardian Chemical Corporation. Acoconut fatty acid diethanolamide, P&G Amide No. 72 marketed by Proctorand Gamble is another useful derivative. The diethanolamide salt oflauryl alcohol sulfate is still another compound that may be used forthe purpose of this invention and is marketed by the DuPont Company asDuponol EP.

The alkanolamines form stable salts with acids such as nitric,hydrochloric, sulfuric, and sulfamic, and these acids are useful for theinitial pH adjustment of the mixture prior to coating. The primary,secondary, or tertiary amines are used on the basis of a few percent byweight of the monomer or monomers employed.

In preparing photo-imaging materials according to the invention, it hasbeen found advantageous to use a hydrophilic colloid as a carrier orbinder for the mixture of monomer, photocatalyst, and the other desiredadditives. Suitable colloids for this purpose include polyvinyl alcohol,gelatin, casein, glue, saponified cellulose acetate, carboxymethylcellulose, starch, and the like. Gelatin is a preferred colloid in thatit not only serves as a binder material but, in addition, serves as abackbone on which the polymer is grafted as it is being formed from themonomer present in the light-sensitive composition.

Although it is preferable to carry out polymerization in a water-basedformulation, an organic solvent system may also be employed. In thiscase, provisions should be made for the presence in the system of asmall quantity of water, e.g., about 0.1 to 5% by weight. The water maybe added as such or may be supplied by incorporation in the reactionmixture of a humectant such as glycerin, trimethylolpropane, ethyleneglycol, diethylene diglycol, or mixtures thereof.

PREFERRED EMBODIMENTS EXAMPLE 1

A monomer solution of the following composition was prepared:

    ______________________________________                                        Acrylamide               90    gms.                                           N,N'-methylenebisacrylamide                                                                            5     gms.                                           Deionized water          60    ml.                                            ______________________________________                                    

To 20 ml. of this solution was added enough of a 10% solution of Arosurf160-E15 coco amine adduct (pH = 9.3) to adjust the pH of the mixture to8.3. A few drops of a 0.2% aqueous solution of methylene blue were thenadded. A 5 ml. portion of this mixture in a 13 × 100 mm. test tube wasthen irradiated with a 500-watt GE Photoflood lamp positioned at adistance of about 25 cm. Polymerization of the monomer ensued after aperiod of 18 seconds. Solid citric acid was next added to the remaining15 ml. of the mixture to adjust the pH to 3.8. No polymer was formedafter a period of 20 minutes when a 5 ml. portion was irradiated asbefore. The pH of the remaining mixture was next adjusted to 6.3 withdilute ammonium hydroxide. Polymer was formed when a 5 ml. portion wasirradiated for 5 minutes as before. Finally, additional dilute ammoniumhydroxide was added to the last 5 ml. portion of the mixture to bringthe pH to the alkaline side. When irradiated as before, polymerizationtook place within a matter of a few seconds.

EXAMPLE 2

To 30 ml. of the monomer composition of Example 1 was added enoughaqueous 50% diethanolamine to adjust the pH to a value of 9.3. Fivedrops of a 0.2% aqueous solution of methylene blue and three drops of a0.2% aqueous solution of rose bengal were added. Polymer was observed toform after irradiating a 5 ml. portion for 35 seconds as in thepreceding example. The remainder of the mixture was then adjusted to apH of 4.35 with a 25% aqueous solution of sulfamic acid. No polymer wasformed after irradiating a 5 ml. portion, as before, for a period of 10minutes. The remaining mixture was finally adjusted by the dropwiseaddition of concentrated ammonium hydroxide to its initial pH value of9.3. When a 5 ml. portion of this mixture was exposed to light as in thepreceding tests, polymer formation took place after 21 seconds.

EXAMPLE 3

A black-colored coating composition was prepared as follows: 9.0 gm.gelatin (inert, high bloom) and 1.5 gm. polyvinyl pyrrolidone (med.visc.) were dissolved in 100 ml. of deionized water. 1.75 gm. of thechlorozincate salt of 1-diazo-2,5-dimethoxy-4-p-tolylmercapto-benzenewere added with stirring to effect complete dissolution of the solid.This was followed by the dropwise addition over a 20 minute period of asolution comprised of 0.7 gm. phloroglucinol, 0.65 gm.8-hydroxyquinoline, and 5.0 gm. N-phenyldiethanolamine dissolved in 100ml. of a 1:1 mixture of denatured ethanol and water. Coupling of thediazonium salt with phloroglucinol formed a black pigment. There wasthen added with continued stirring a solution of 4.2 gm. acrylamide and0.8 gm. N,N-'-methylenebisacrylamide dissolved in 50 ml. of deionizedwater. Five ml. of 20% aqueous solution of sodium dodecyl benzenesulfonate were then added, followed by 2 ml. of a 0.5% aqueous solutionof methylene blue. Apparently due to the acidity of the diazonium saltemployed, the pH of the completed formulation was 4.5 and furtherdownward adjustment was not required.

The composition was divided into two parts. One was retained ascomposition A and the other was modified by the addition of 2.5 ml. of a0.2% aqueous solution of rose bengal. This latter solution wasdesignated as composition B. Each formulation was then coated on asubbed, matte surface, polyester film base, commonly used for draftingwork, by means of a vertical loop meniscus coater at a speed of about 4cm. per second. Both coatings were allowed to dry in darkness.

A section of coating A was fumed with ammonia for 3 minutes by placingit coating side downward over a beaker containing a few ml. ofconcentrated ammonium hydroxide. After placing it in contact with a linenegative, it was exposed in a printing frame for about 3 minutes to thelight of a 500-watt GE Photoflood lamp positioned at a distance of about30 cm. The exposed layer was then washed with water at a temperature ofabout 30° C with mild finger rubbing whereupon a positive image wasobtained. When another section of the coating was exposed as before, butwithout prior fuming with ammonia, there was no discernible image afterthe washout step.

EXAMPLE 4

A section of coating B prepared in Example 3 was given a 4 minuteimagewise exposure without prior fuming with ammonia. There was nodiscernible image after the washout step.

EXAMPLE 5

Another section of coating B was fumed with ammonia for 1.5 minutes. Agood positive print was obtained after a 3 minute exposure and wash withwater at about 30° C.

EXAMPLE 6

A section of coating B was exposed imagewise to ambient fluorescentillumination for 16 hours without prior fuming with ammonia. There wasno discernible image after the washout step.

EXAMPLE 7

Another section of coating B was fumed with ammonia for 1.5 minutes andthen given a 3 minute imagewise exposure as before. The sheet was thenheated in a 40° C oven for 1 minute. Thereafter, the sheet was given a 3minute exposure of a different image using the same light source. Only asingle image corresponding to the first exposure was evident afterwashout processing.

EXAMPLE 8

A section of coating B was fumed with ammonia as in Example 7, given a 3minute imagewise exposure and then heated in a 40° C oven for oneminute. The sheet was then fumed with ammonia once again and given adifferent 3 minute imagewise light exposure as before. Two differentimages corresponding to the separate exposures became evident after thewashout step.

EXAMPLE 9

A portion of composition B was adjusted to a pH of 5.5 with dilutesodium hydroxide and then coated on film base as in Example 3. Afterdrying, a section, without prior fuming with ammonia, was given a 4minute imagewise exposure. A faint image, as compared with nodiscernible image under conditions of Example 4, was obtained after thewashout step.

EXAMPLE 10

A composition of the following formulation was prepared in the generalmanner of Example 3:

    ______________________________________                                        Gelatin (inert, high bloom)                                                                           9.0    g.                                             Polyvinyl pyrrolidone (med. visc.)                                                                    1.5    g.                                             Deionized water         100.0  ml.                                            Acrylamide              4.2    g.                                             N,N'-methylenebisacrylamide                                                                           0.8    g.                                             Deionized water         50.0   ml.                                            Coco amine adduct (160-E 15)                                                                          50.0   ml.                                            Methylene blue (0.5% aq. sol.)                                                                        1.0    ml.                                            Rose bengal (0.5% aq. sol.)                                                                           1.0    ml.                                            Trimethylolpropane (50% aq. sol.)                                                                     2.0    g.                                             Glycerin                2.0    g.                                             ______________________________________                                    

The resulting mixture was substantially colorless, lacking thepigmenting components of Example 3, and exhibited a pH of about 7.2.Sufficient sulfamic acid (10% sol.) was added to reduce the pH of thecomposition to about 4.2 and a coating of about 125 microns wetthickness was applied to a subbed polyester film and dried in darknessovernight.

The sheet material thus prepared was employed over the period of a dayto prepare a number of images in the following manner:

A section of the coated composition of the material was fumed withammonium hydroxide vapors for about 60 seconds and the section was thenexposed for about 30 seconds to a first light image from a 500-wattphotoflood lamp at a distance of about 30 cm. The exposed sheet was thenplaced in the substantial darkness of a file cabinet for about 6 hoursafter which it was removed and immediately, without ammonia fuming, wassimilarly exposed in a different section to a second light image fromthe same source. After another period of simulated file storage, thesheet was again fumed in a third section with the ammonia vapors andthen exposed to a third light image under the previous conditions. Onceagain the sheet was placed in file storage at ambient conditions for theremainder of the day.

Subsequently, the exposed sheet was viewed in a Schlieren optical systemin the light of He-Ne laser. The images corresponding to the first andthird light exposures were distinctly visible, yet there was no apparentimage of the second exposure. Still later, the sheet was subjected towashout development with water and the presence only of the first andthird images was confirmed.

What is claimed is:
 1. The method of forming a polymeric image whichcomprises:a. providing photo-imaging material comprising a support and acoating thereon of a composition comprising an ethylenically unsaturatedpolymerizable compound, a photo-initiator comprising the combination ofa photoreducible dye and an alkanolamine, and sufficient acid tonormally maintain said composition at a pH value below about 4.5,thereby rendering said material substantially insensitive to visiblelight; b. alkalizing at least a section of the coated composition ofsaid material to render the pH value of said composition section greaterthan about 7.0; and c. exposing imagewise to visible light saidalkalized composition section, thereby effecting polymerization of saidcomposition in light-exposed areas.
 2. The method of forming a stablepolymeric image which comprises:a. providing photo-imaging materialcomprising a support and a coating thereon of a compositioncomprising:1. an ethylenically unsaturated polymerizable compound,
 2. aphoto-initiator comprising the combination of a photoreducible dye andan alkanolamine, and;
 3. sufficient acid to normally maintain saidcomposition at a pH value below about 4.5, thereby rendering saidmaterial substantially insensitive to visible light; b. alkalizing atleast a section of the coated composition of said material to render thepH value of said composition section greater than about 7.0; c. exposingimagewise to visible light said alkalized composition section, therebyeffecting polymerization of said composition in light-exposed areas; andd. fixing said polymeric image by effecting a reduction in the pH valueof the alkalized section of said composition to below about 4.5.
 3. Amethod according to claim 2 wherein said alkalizing step comprisesexposing said composition section to alkaline vapors, and said fixingstep comprises effecting removal of said vapors from said composition.4. A method according to claim 3 wherein said alkaline vapors compriseammonia.